Heat-sensitive copying-paper



Oct. 27, 1959 R. OWEN 2,910,377

HEAT-SENSITIVE COPYING-PAPER Filed June 28, 1956 /4 #7 m N W [NVENTUR P/CHARD OWEN Arron/v2: Y5

United States Patent HEAT-SENSITIVE COPYHlG-JAPER Richard Owen, Brooklyn Center, Minn., assignor to Minnesota Mining & Manufacturing Company, St. Paul, Minn., a corporation of Delaware.

Application June 28, 1956, Serial No. 594,540

11 Claims. (Cl. 117-36) This invention is concerned with improvements in heat-sensitive copying-paper useful in preparing copies of printed matter by procedures, such as those described in Miller Patent No. 2,740,896, involving placing a printed or other graphic original in heat-conductive association with the heat-sensitive copying-paper and then briefly exposing the graphic original to intense radiant energy. The resulting elevated-temperature pattern produces in the copying-paper a visible direct copy of the graphic original. The present invention provides a stable heat-sensitive copying-paper based on a new principle of operation and capable of providing duplicate copies having improved appearance as well as other advantages.

For convenience in describing the novel copying-paper and its application to the copying of documents, the product and procedure are illustrated in the accompany ing drawing, in which:

Figure 1 schematically illustrates in cross-section one method of copying a graphic original;

Figure 2 similarly illustrates an alternative method; and

Figure 3 illustrates in cross-section a preferred form of heat-sensitive copying-paper.

A commercially available thermo-copying machine widely employed for copying oflice records on heat-sensitive copying-paper includes as the source of radiant energy a high-intensity infrared lamp such as a GE T-3 lamp having a linear coiled filament supported within a reflector which focuses the radiation in a narrow beam across the sheet as the latter is moved past the lamp assembly. Such a lamp draws 1350 watts at 280 volts to provide a color temperature of about 2800 K. Irradiation and copying of a conventional letter-size sheet requires about 16 seconds. A non-fading copy is directly produced by such irradiation and without any subsequent processing such as developing, fixing, washing, treatment with fumes or vapors, or any other treatment.

In Figure 1, radiant energy from source is directed through heat-sensitive copying-paper 11 to printed original 12; Energy is selectively absorbed in printed areas 13 and released in the form of heat energy, causing the occurrence of a visible change in corresponding areas 14 of the heat-sensitive layer 15 of sheet 11. The procedure is known as front-printing.

In Figure 2 the radiant energy from source impinges directly on the printed surface of the graphic original 22, and the resulting release of heat energy causes visible change in the heat-sensitive layer on support 26 at areas 24 corresponding to the printed and radiationabsorptive areas 23 of the original. The procedure has been termed back-printing.

The copying-sheet 11 of Figure 1 consists of a heatsensitive layer 15 on a carrier 16, which may be a thin paper. The sheet must transmit sufiicient of the radiant energy to provide the required release of heat at the printed areas 13 but is preferably sufiiciently visibly opaque to provide high contrast in the copy.

Still greater accuracy of copy is obtained by inverting against a metal test bar heated to about -120" C the copying-paper 11 so that the heat-sensitive layer 15 is more closely associated with theprinted surface of the graphic original 12. In this case a visibly transparent carrier 16 permits viewing of the copy areas 14 through the carrier and in the same direction as the original.

The front-printing process of Figure 1 is applicable to a Wide variety of originals and to radiation-transmissi've copy-paper, whereas back-printing is most effective with originals printed on relatively thinheattransmissive paper and the copy-paper may be fully opaque to the radiation employed.

Figure 3 illustrates a further variation in heat-sensitive copying-papers in which the heat-sensitive layer 35 is applied to a carrier web 36 and is covered with a protective layer 37. Either or both of web 36 and layer 37 may be visibly transparent, but in a preferred con-. struction for front-printing operations the web 36 is transparent while the thin protective layer 37 is visibly opaque and provides high visual contrast for the visibly changed areas of the. heat-sensitive layer.

The actual temperatures developed in such heat-sensitive papers by these various techniques have not been directly measured. However it has been observed that papers which change visibly when momentarily pressed or at most to about 150 C., become similarly altered under the irradiation techniques just described. On the other hand, papers which do not change on brief pressure contact with the test bar at temperatures much greater than about 150 C. are found to be ineffective as heatsensitive copying-papers when tested in copying-machines as described hereinbefore. Since heating to such high temperatures, particularly when prolonged or frequently repeated, is likely to cause degradation or deterioration of the printed page forming the original of which acopy is desired, copy-sheets are ordinarily required to be visibly heat-sensitive at temperatures below about 150 C. and preferably at temperatures below about C.

Heat-sensitive copying papers, suitable for the copying of printed and other graphic originals by methods just described have previously been developed, and such products are described in Miller et al. Patents Nos. 2,663,6542,663,657, in Taylor et al Patent No. 2,668,126, and in Clark et a1. Patent No. 2,710,263. The sheet materials of the present invention operate through a different mechanism than do these prior art materials and provide a number of advantages thereover, as will be pointed out hereinafter in connection with illustrative but non-limiting examples.

Like the earlier heat-sensitive copying-papers, those of the present invention are suitable for making direct, high contrast, clear detail, permanent copies of typewritten, printed, and other graphic subject-matter. The copypaper does not adhere to nor deface the graphic original or the thermoprinting apparatus. It is stable at normal room and storage temperatures, and in particular may be stored in the light without visible change either before or after heat-copying. The present invention additionally provides for the utilization of new classes of visibly heatsensitive compositions, and in a preferred modification makes available a white sheet which converts to a dense black at the heatedimage areas, thus closely duplicating the appearance of the usual typewritten or printed original.

These and other advantageous properties are secured, in accordance with the principles of the present invention, by employing as the heat-sensitive coating an intimate mixture of components including an oxidizing agent and a reducing agent which when sufiiciently heated, as in the thermocopying process described, undergo an oxidationreduction type reaction resulting inthe formation of one or more visibly distinct reaction products. These components are normally employed in conjunction with a water-resistant film-forming binder, the combination being in the form of either a self-supporting thin film or a thin coating on a suitable paper or other carrier web. The reactants are present in amounts sufficient to provide adequate visible change on heating, and the binder maintains the reactants in the required intimate association and in position on the paper backing. Additional components, both inert and chemically reactive, may be included to improve the appearance or handling properties of the sheet, or to provide a more intense or differently colored heat-image, or for other purposes.

, The following specific examples further illustrate the invention.

Example 1 A mixture of equal mol percent of silver behenate and behenic acid is prepared by reacting together one mol of silver nitrate and two mols of sodium behenate, made from commercial behenic acid, in aqueous medium and in the presence of nitric acid. The resulting waterinsoluble precipitate is recovered on a filter and iswashed and dried to produce a fine powder. The powder fuses at about 135 C. and melts to a liquid at about 175 C. Silver behenate and behenic acid, prepared separately from the same sodium behenate, melt at about 220 C. and about 70 C. respectively.

Six parts by weight of the powder is dispersed in a solution of one part of polystyrene resin in 43 parts of ethyl acetate by grinding in a ball mill for 8-16 hours to provide a'uniform and very fine dispersion. The silver behenate is substantially completely insoluble, the behenic acid only slightly soluble, in the solvent employed. Commercial behenic acid contains small proportions of other long chain fatty acids.

One and one-half parts by weight of protocatechuic acid (3,4-dihydroxybenzoic acid, melting at about 199 C.), is separately dissolved, together with 8 parts of polystyrene resin and 2 parts of diphenyl phthalate, in 38.5 parts of ethyl acetate.

The two mixtures as above prepared are mixed together, producing a coating composition containing 9.5% non-volatile materials. This composition is coated on 25 lb. map overlay tracing paper, a thin, non-porous, lightly calendered and substantially transparent carrier web, at a thickness sufficient to provide approximately one-half gram of the non-volatile components per square foot of coated area. This corresponds to about 25 milligrams of silver metal per square foot of area.

After drying, the coated sheet is in condition for use.

as a heat-sensitive copying-paper, and when used as hereinbefore indicated is found to yield dense black reproductions on a translucent white background. The sheet is stable on prolonged storage and on exposure to light, and has excellent handling properties.

The two mixtures may alternatively be separately applied to the paper base, with intermediate drying. The reactants are intimately associated at the interface, and the sheet produces visible but somewhat less distinct copy when tested as a heat-sensitive copying-paper.

The same solutions may also be separately coated and dried on separate thin papers, the coated surfaces pressed firmly together, and the resulting composite employed in making a copy by the processes here described followed by separation of the two coated surfaces. Copy thus formed is not susceptible to modification by further heating.

Example 2 The copying-paper of Example 1 is further coated, over the heat-sensitive coating, with a thin layer of a mixture of 3 parts by weight of styrene-isobutylene copolymer (Parapol S-50 resin), 66 parts of hexane, 30 parts of zinc oxide and one part silica gel (Santocel C), the mixture being ball milled for 8-26 hours to provide a viewed from the uncoated side, i.e., through the transparent paper.

Omitting the opacifying pigments from the coating composition provides a clear transparent protective layer which prevents surface marring and otherwise protects the heat-sensitive layer while permitting it to be seen through the coating. Such a structure is of particular value where an opaque or colored paper or other backing member is desired.

- Example 3 A suspension of 6 parts by weight of the silver behenate: behenic acid mixture of Example 1 is dispersed in a solution of 1.5 parts of polystyrene resin in a mixture of 16 parts heptane and 26.5 parts acetone by ball milling to a smooth dispersion. Separately, 1.5 parts of methyl gallate, 0.1 part of 2,3-dihydroxybenzoic acid, and 0.2 part of phthalic anhydride are stirred into a solution of 11.6 parts of polystyrene resin in 13.6 parts heptane and 23 parts acetone. The two solutions are blended together and the blend coated smoothly on map overlay tracing paper, the dried coating weighing 0.5 gram per square foot, equivalent to about 21 milligrams of silver.

Trace amounts of inhibitors, e.g., citric acid, are preferably included with the phthalic anhydride to prevent reaction and darkening of the solution prior to coating.

The sheet is then further coated with 0.5-1.5 grams per square foot dry weight, of a smooth dispersion of 30 parts of zinc oxide, 3 parts of Parapol S-SO resin, 1 part of Santocel C filler, and 0.08 part of phthalic anhydride in 66 parts of heptane.

The product is white in appearance. It does not discolor on prolonged storage at normal room and storage temperatures, nor on exposure to sunlight. When pressed against a printed page and the latter intensely irradiated as hereinbefore described, the sheet darkens to a lustrous black appearance at the heated areas and provides a clear, sharp duplicate of the original printed matter.

Example 4 Approximately equimolar amounts of silver acid phthalate and a mixture of isomeric forms of 4-methoxy, 1- hydroxy-dihydronaphthalene available from Universal Oil Products Co. as UOPl524l22B Antioxidant) are combined with polystyrene resin binder in solution in a mixture of heptane and acetone, and the composition is coated on paper and dried, as described in'connection with Example 3. A sheet is produced which, when exposed to a heat-pattern as already described, yields a greenish image at moderate exposure and a brownish image under increased exposure. The stability of the sheet is improved by incorporation of small amounts of phthalic acid.

Example 5 Mercuric behenate is combined with methyl gallate by procedures as already described to provide an initially colorless stable coating which produces a brownish green image when employed as a heat-sensitive coppying-paper as herein described. In this example the metal ion is reduced to a lower valence state without visible change, whereas the methyl gallate is oxidized to a visibly diflerent compound providing the visible image.

Example 6 Ferric stearate is employed as the oxidizing agent and the 4rgethoxy, 1-hydmXy-dihydronaphthalene mixture of. Example 4 as the reducing agent. The resulting stable heat-sensitive copy-paper yields a blue image on a butt background.

Example 7 Gold stearate and 2,5-dihydroxybenzoic acid produce a very pale purple stable heat-sensitive copy-paper which is converted to a purplish black image in the thermo copying process. The initial slight color is believed to be due to impurities in the gold stearate. The image areas are at least in part composed of metallic gold.

Example 8 A mixture of cerium stearate and the 4-methoxy, lhydroxy-dihydronaphthalene reducing agent of Example 4 provides a colorless stable heat-sensitive copy-paper yielding a. blue image.

Example 9 Example 10 Tetradecyl amine molybdate and hydroquinone are mixed with resinous binder solution and coated on a carrier web, all as hereinbefore described, to provide an uncolored stable heat-sensitive copy-paper which produces red-brown image areas,

Example 11 Mercuric behenate is employed as an oxidizing agent and hydroquinone as a reducing agent. In order to provide for a more intense image than is obtained with these components alone, there are also included an oxidationresistant mixture of tris(p-diethylaminophenyl) methane and a slight excess of dodecyl amine. The faintly greenish heat-sensitive copy-paper made with such composition, when subjected to the copy-process as herein described, initially is only slightly changed in appearance. After standing in the air for a few minutes, or at most for a few hours, a dark blue image develops at the heated areas. In probable explanation of thin behavior, it is suggested that an immediate initial oxidationreduction reaction occurs between the mercuric behenate, .Which is reduced to mercurous behenate and free behenic acid, and the hydroquinone, which is oxidized to quinone, causing a slight visible change. The behenic acid neutralizes the dodecyl amine, leaving the tris(p-diethylaminophenyl)methane in condition for oxidation, and the latter is then slowly oxidized to a strongly colored product by reaction with oxygen absorbed from the air. This proposed explanation is supported by the observed fact that best results in terms of image visibility are obtained when the components are stoichiometrically proportioned in accordance with the requirements of such theory, and when the coating is designed to permit access of air to the reactants. It is also observed that the presence of free behenic acid in the mercuric behenate component decreases the stability of the coated sheet and results in a slow change from the initially pale green color to a light blue shade.

Each of the several specific examples provides an effective and highly useful stable heat-sensitive copyingpaper, regardless of the specific mechanism involved.

However the reactions involving reduction of a metal salt to free metal have certain advantages and are presently preferred. These reactions, represented in particular by Examples 1, 3, 4, and 7, employ components which are readily available or readily prepared and in substantially colorless form, and the resulting heat-images in general are extremely dense and readily visible even with relatively low concentrations of reactants.

Silver behenate, employed in Examples 1 and 3, is a preferred example of an organic acid salt of a reducible metal which, as has been shown by analysis of the resulting image areas, is reduced to the free metal during the oxidation-reduction reaction. The silver content is readily reduced to metallic silver. The compound is colorless, visibly stable toward light, insoluble in many volatile liquid vehicles, and moisture-resistant. It is produced in the desired physical form without difficulty and at reasonable cost.

Silver stearate, has been successfully substituted for silver behenate, and silver salts of many other organic acids have also been found useful in these heat-sensitive compositions and copying-papers. A partial list of such organic acids includes oleic, lauric, hydroxystearic, acetic, phthalic, terephthalic, butyric, m-nitrobenzoic, salicylic, phenylacetic, pyromellitic, p-phenylbenzoic, undecylenic, camphoric, furoic, acetamidobenzoic and o-aminobenzoic.

Similarly, other reducing agents have been substituted for the specific materials of Examples 1 and 3, although the latter at present are believed to provide the best over-all combination of properties and are preferred.

Protocathechuic acid, when used in conjunction with the silver behenate: behenic acid combination of Example 1, provides a system which is initially free of color, is stable at normal room and storage conditions, reacts rapidly at temperatures well within the range of the apparatus and method employed, and produces a lustrous black image area having maximum contrast against the white background of the copying-paper. Methyl gallate alone is not quite as effective, but the addition of suitable modifiers as in Example 3 produces an eminently satisfactory stable heat-sensitive copyingpaper. Copying-paper made with propyl gallate and silver behenate reacts effectively at temperatures in the neighborhood of l00 C. as determined on a heated metal test bar, and produces good readable copy; but the initially greenish colored image area turns brown and loses some of its density with additional heating. However in each instance the combination of the silverreducing agent with the dispersion of organic silver salt produces a visibly heat-sensitive coating which when briefly heated well above its normal stable storage temperature reacts to liberate free silver and to produce a visible change.

Reducing agents which have been found useful in providing such combinations include the following; pyrogallol; 4-azeloyl-bis-pyrogallol; 4-stearoyl pyrogallol; galloacetophenone; di-tertiary-butyl pyrogallol; gallic acid anilide; methyl gallate; ethyl gallate; normaland iso-propyl gallate; butyl gallate; dodecyl gallate; gallic acid; ammonium gallate; ethyl protocatechuate; cetyl protocatechuate; 2,5 -dihydroxy benzoic acid; l-hydroxy-Z- naphthoic acid; Z-hydroxy, 3-naphthoic acid; phloroglucinol; cathechol; 2,3-naphthalene diol; 4-lauroyl catechol; sodium gallate; protocathechualdehyde; 4- methyl esculetin; 3,4-dihydroxy benzoic acid; 2,3-dihydroxy benzoic acid; hydroquinone; 4,4'-dihydroxy biphenyl; 3,4-dihydroxyphenylacetic acid; 4(3,4-dihydroxyphenylazo)benzoic acid; 2,2'-methylene bis-3,4,5- trihydroxybenzoic acid; benzidine; orth-oand paraphenylene diam-inc; tetrarnethyl benzidine; 4,4,4' -diethylamino triphenylmethane; 0-, m-, and p-aminobenzoic acids; alpha and beta naphthols; 4-methoxy, l-hydroxydihydronaphthalene; and tetrahydroquinoline.

While the invention is not necessarily to be limited thereto, it may be noted that many of the most effective reducing agents, and particularly the silver-reducing agents, are aromatic organic reducing agents which are oxidizable with loss of aromaticity while still retaining a ring structure.

Observed changes in reaction rate of these papers with change in temperature in the neighborhood of the operating temperature suggest that the reaction should proceed at a measurable rate under storage conditions. Surprisingly, however, such heat-sensitive copying-paper is visibly stable and unchanged even under the influence of light and on prolonged storage at room temperatures. Nor is light-sensitivity required, since copying-papers have been made and tested, according to principles here defined, under conditions of complete darkness, and have been shown to be completely operable as heat-sensitive copying-papers under such conditions. Where the light sensitivity of the salt may interfere with stability of the coated sheet, visibly transparent filter layers may be applied over the light-sensitive layer to protect the latter against actinic radiation. Various dyes are known which are effective for this purpose.

Combinations of silver salt, reducing agent and binder in the absence of the free organic acid as used in Example 1 have provided useful heat-sensitive coatings which in the form of heat-sensitive copying-papers were sufiiciently stable and produced adequate visible change for many non-critical applications. Stability towards exposure to light is improved by selecting highly purified materials; freedom from halides and sulfides is particularly important in the case of compositions involving silver salts. Stability towards both light and moisture is improved by the inclusion of free organic acid as provided in the formulas of Examples 1 and 3. Behenic acid and similar water-insoluble organic acids are particularly effective. The proportions of free acid and silver salt may vary widely and the free acid may be omitted entirely; but equimolar proportions as shown in Example 1 are preferred and best results are ordinarily obtained within the approximate acid: salt molar ratios of 1:3 to 3:1. 1

An amount of silver salt sufficient to represent at least about 20 milligrams of silver metal per square foot of sensitive layer is found to produce fully adequate image density in the sheet material of Example 1. The amount of reducing agent in such sheet accordingly should likewise be at least sufficient to reduce a quantity of silver ion equivalent to about 20 milligrams of silver per square foot of sensitive layer. Since some reagents, particularly some of the organic reducing agents, may be volatilized to some slight extent during prolonged storage prior to use, such components may be added in greater than stoichiometrically equivalent proportions; but equivalent proportions are normally preferred. Less effective but still visibly distinct image areas may be produced at still lower concentrations.

Other additive materials have been used for various special effects. The addition of very small amounts of phthalic anhydride in the formula of Example 3 has been found to result in formation of a black rather than a brownish image on the application of heat; a darker image is also obtainable by increasing the thickness of the reactive layer or the effective concentration of silver. A mixture of catechol and benzoic acid with the silver behenate produces an image area having a greenish tinge, the color varying with the temperature employed; catechol, being somewhat volatile, is less desirable where prolonged storage, or contact with reactive materials, is a requirement. The addition of ammonium stearate to a gallic acid: silver'behenate combination results in a sheet which darkens slowly on prolonged storage but with which images of a variety of colors have been obtained, the color varying with the temperature and with the rate of heating employed.

Stability of the heat-sensitive composition is influenced adversely by the presence of water. It is therefore desirable to employ resinous binders which are substantially non-absorptive of moisture and olfer maximum protection to the reactive components. Polystyrene resins are particularly effective. Less moisture-resistant binders r 8 a may frequently be improved by the incorporation of small amounts of waxes or other moisture-proofing materials. Moisture-resistant surface coatings are also helpful;

Within the limitations imposed by the requirements of water and moisture resistance, transparency or opacity, permanence, handleability, light stability, operating temperatures, flexibility, etc., as hereinbefore indicated, it will be apparent that a considerable number of specifically different but fully equivalent film-forming binder materials, pigments and fillers, solvents and diluents, resins, plasticizers, fibrous and non-fibrous supporting webs, and various other auxiliary materials may equally well be employed,

Specifically, the map overlay tracing paper has been replaced by other fibrous and non-fibrous sheet materials such as Monarco paper and cellophane, and the coatings are found to be operable also when applied to wood, leather, fabrics, and many other substrates, or as selfsupporting films. Ethyl cellulose is useful as a filmforming binder material although somewhat less moisture-resistant than polystyrene resin. Where plasticizers for the resinous binder are found to be desirable, phosphates and phthalates, having low water absorption properties, are superior to glycols and their esters. Zinc sulfide, titanium dioxide, diatomaceous earth, and numerous other pigments and fillers are useful in conjunction with the protective surface coating; metal powders and carbon black, for example, may be used in substantial proportions in back-printing copying-papers, and have been found useful in very small proportions in the surface coatings of front-printing papers, e.g., to provide somewhat higher sheet temperatures. Pigments and other particulate materials such as glass cullet or sphericles may also be added to the heat-sensitive coating for special purposes where desired.

Silver behenate, and silver stearate to a somewhat lesser extent, is substantially insoluble in ethyl acetate and in heptane-aectone mixtures at normal room temperatures and is mixed with the other components of the heatsensitive coating in the form of a dispersion or suspension in such solvents or mixtures, as indicated in Examples 1 and 2. Most salts having a relatively high degree of solubility in a particular vehicle have been found to react with the reducing agent to an extent sufiicient to cause undesirable darkening of the liquid mixture and of the dried coating. In such cases it is ordinarily possible to select for the vehicle a volatile liquid in which either the oxidizing agent, or the reducing agent, or both are substantially completely insoluble. In other instances the components may be dissolved together in a mutual solvent held at reduced temperature, and then coated and dried without heating, to provide useful heat-sensitive copying-paper.

What is claimed is as follows:

1. A heat-sensitive copying-paper for making clear and sharp copies of graphic subject matter by methods involving brief application of a heat-pattern corresponding to said graphic subject-matter as herein described, said copying-paper being visibly stable at normal room and storage temperatures and under exposure to light, said copying-paper including a visibly heat-sensitive layer containing, in intimate association, one mol of silver behenate, about one mol of behenic acid, and and about one-half mol of protocatechuic acid.

2. A heat-sensitive copying-paper for making clear and sharp copies of graphic originals by methods involving brief application of a heat-pattern corresponding to said graphic original as herein described, said copyingpaper being visibly stable at normal room and storage temperatures and under exposure to'light, said copyingpaper comprising a paper base and a visibly heat-sensitive coating comprising, uniformly dispersed in intimate association in a water-resistant binder, one mol of silver behenate, about one mol of behenic acid, and about onehalf mol of protocatechuic acid.

3. A heat-sensitive copy-sheet for making a clear and sharp reproduction of a graphic original by methods involving brief application of a heat-pattern corresponding to said graphic original as herein described, said copysheet being visibly stable under normal storage conditions and being rapidly permanently visibly changed on heating to a conversion temperature of the order of about 90-15 C., said copy-sheet including a visibly heat-sensitive layer containing, in intimate association, (1) a normally solid organic acid salt of a noble metal, and (2) a cyclic organic reducing reagent for the noble metal ions, having an active hydrogen atom attached to an atom, se lected from the class of oxygen, nitrogen and carbon atoms, directly attached to an atom of the cyclic ring, said reducing reagent being further characterized as causing the reduction of silver ion and precipitation of metallic silver on being dissolved at moderate temperature in a solution of aqueous silver nitrate in an organic solvent, each of reagents (l) and (2) being stable against decomposition when separately heated to said conversion temperature.

4. A heat-sensitive copy-sheet for making a clear and sharp reproduction of a graphic original by methods involving brief application of a heat-pattern corresponding to said graphic original as herein described, said copysheet being visibly stable under normal storage conditions and being rapidly permanently visibly changed on heating to a conversion temperature of the order of about 90-150 C., said copy-sheet comprising a thin carrier Web and a heat-sensitive coating thereon including (1) a filmforming binder, (2) a Water-insoluble normally solid silver salt of an organic acid, and (3) an aromatic organic reducing reagent for the silver ions, having an active hydrogen atom attached to an atom, selected from the class consisting of oxygen, nitrogen and carbon atoms, directly attached to an atom of the aromatic ring, said reducing reagent being further characterized as causing the reduction of silver ion and precipitation of metallic silver on being dissolved at moderate temperature in a solution of aqueous silver nitrate in an organic solvent, each of reagents (2) and (3) being stable against decomposition When separately heated to said conversion temperature.

5. A heat-sensitive copy-sheet for making a clear and sharp reproduction, in the form of black infraredabsorptive image areas on white background areas, of a black-on-white graphic original by methods as described in the accompanying specification and involving brief application of a heat-pattern corresponding to the black areas of said original, said copy-sheet being visibly stably white under normal storage conditions and being rapidly permanently converted to black on heating to a conversion temperature of the order of about 90-150" C., said copysheet comprising a paper carrier web and a heat-sensitive coating thereon including (1) a film-forming binder, (2) a Waterinsoluble, normally solid, substantially colorless silver salt of an organic acid, and (3) a substantially colorless aromatic organic reducing reagent for the silver ions, having a hydroxyl group attached directly to an atom of the aromatic ring, said reducing reagent being further characterized as causing the reduction of silver ion and precipitation of metallic silver on being dissolved at moderate temperature in a solution of aqueous silver nitrate in an organic solvent, each of reagents (2) and (3) being stable against decomposition when separately heated to said conversion temperature.

6. A heat-sensitive copy-sheet for making a clear and sharp reproduction of a graphic original by methods as described in the accompanying specification and involving brief application to said copy-sheet of a heat-pattern corresponding to said graphic original, said copy-sheet being visibly stable under normal storage conditions and being rapidly permanently visibly changed on heating to a conversion temperature of the order of about 9,0-150 C., said copy-sheet comprising a paper carrier Web and a heat-sensitive coating thereon including (1) a moistureresistant resinous film-forming binder, (2) a Water-insoluble normally solid organic acid silver salt, and (3) an aromatic organic reducing reagent for the silver ions, having an active hydrogen atom attached to an atom, selected from the class consisting of oxygen, nitrogen and carbon, directly attached to an atom of the aromatic ring, said aromatic reducing reagent being oxidizable with loss of aromaticity while still retaining a ring structure and being further characterized as causing the reduction of silver ion and precipitation of metallic silver on being dissolved at moderate temperature in a solution of aqueous silver nitrate in an organic solvent, each of reagents (2) and (3) being stable against decomposition When separately heated to said conversion temperature.

7. A heat-sensitive copy-sheet as defined in claim 4 in which the silver salt and the aromatic organic reducing reagent are present in approximately equimolar proportions.

8. The heat-sensitive copy-sheet of claim 4 in which the heat-sensitive coating is covered by an outer thin protective layer and at least one of said carrier web and said outer protective layer are transparent.

9. A heat-sensitive copy-sheet for making a clear and sharp reproduction of a graphic original by methods as described in the accompanying specification and involving brief application to said copy-sheet of a heat-pattern corresponding to said graphic original, said copy-sheet being visibly stable under normal storage conditions and in the presence of moisture and being rapidly permanently visibly changed on heating to a conversion temperature of the order of about 150 C., said copy-sheet comprising a paper carrier web and a heat-sensitive coating thereon including (1) a moisture-resistant resinous filmforming binder, (2) a water-insoluble normally solid silver salt of a solid organic acid, (3) a quantity of a solid free organic acid, and (4) an aromatic organic reducing reagent for the silver ions, having an active hydrogen atom attached to an atom, selected from the class consisting of oxygen, nitrogen and carbon, directly attached to an atom of the aromatic ring, said reducing reagent being further characterized as causing the reduction of silver ion and precipitation of metallic silver on being dissolved at moderate temperature in a solution of aqueous silver nitrate in an organic solvent, each of reagents (2) and (4) being stable against decomposition when separately heated to said conversion temperature.

10. A heat-sensitive copy-sheet for making a clear and sharp reproduction of a graphic original by methods as described in the accompanying specification and involving brief application to said copy-sheet of a heat-pattern corresponding to said graphic original, said copy-sheet be ing visibly stable under normal storage conditions and in the presence of moisture and being rapidly permanently visibly changed on heating to a conversion temperature of the order of about 90-150 C., said copy-sheet comprising a paper carrier Web and a heat-sensitive coating thereon including (1) a moisture-resistant resinous filmforming binder, (2) a water-insoluble normally solid silver salt of a solid organic acid, (3) a quantity of said organic acid, and (4) an aromatic organic reducing reagent for the silver ions, having an active hydrogen atom attached to an atom, selected from the class consisting of oxygen, nitrogen and carbon, directly attached to an atom of the aromatic ring, said reducing reagent being further characterized as causing the reduction of silver ion and precipitation of metallic silver on being dissolved at moderate temperature in a solution of aqueous silver nitrate in an organic solvent, each of reagents (2) and (4) being stable against decomposition when separately heated to said conversion temperature, the molal proportions of silver salt, aromatic organic reducing reagent, and free organic acid being approximately 1:1:Vs-3.

11. The heat-sensitive copy-sheet of claim 10 in which the paper carrier web is transparent and the heat-sensitive coating is covered by an outer thin opaque pigmented 5 protective layer.

References Cited in the file of this patent UNITED STATES PATENTS Miller Dec. 22, 1953 Miller Dec. 22, 1953 Miller Apr. 3, 1956 Shoemaker June 5, 1956 

1. A HEAT-SENSITIVE COPYING-PAPER FOR MAKING CLEAR AND SHARP COPIES OF GRAPHIC SUBJECT MATTER BY METHODS INVOLVING BRIEF APPLICATION OF A HEAT-PATTERN CORRESPONDING TO SAID GRAPHIC-MATTER AS HEREIN SESCRIBED, SAID COPYING-PAPER BEING VISIBLY STABLE AT NORMAL ROOM AND STORAGE TEMPERATURES AND UNDER EXPOSED TO LIGHT, SAID COPYING-PAPER INCLUDING A VISIBLY HEAT-SENSITIVE LAYER CONTAINING, IN INTIMATE ASSICIATION ONE MOL OF SILVER BEHENATE, ABOUT ONE MOL OF BEHENIC ACID, AND AND ABOUT ONE-HALF MOL OF PROTOCATECHUIC ACID. 